Little is known about the vascular dysfunction and injury that results from temporary retinal ischemia. We hypothesize that retinal ischemia is followed acutely by hypoperfusion, blood- retinal barrier disruption, loss of reactivity to endothelium- dependent dilators, and an inflammatory response characterized by leucocyte-endothelial adherence, all of which are interdependent factors that contribute importantly to the overall injury sustained by this tissue. Using in vivo and in vitro retinal ischemia models, we will test the related hypotheses that: l) oxygen free radicals produced by endothelial cells and leukocytes during postischemic reperfusion are responsible for these manifestations of vascular dysfunction; 2) nitric oxide can protect endothelium by inactivating oxygen free radicals, preserving vascular reactivity, and limiting leukocyte adherence; and 3) the cytokine TNF alpha is intimately involved in mediating endothelial injury secondary to promoting endothelial adhesion molecule expression and leukocyte infiltration. We will examine the efficacy of different oxygen free radical inhibitors and scavengers. nitric oxide donors, and TNF alpha inhibitors, in attenuating postischemic hypoperfusion, vascular leakage, impaired reactivity, adhesion molecule expression, and leukocyte-endothelial interactions. In vivo, we will use fluorescence videoangiography in piglets to measure changes in retinal blood flow during post- ischemic reperfusion. Impaired responses to intravitreally delivered nitric oxide- dependent dilators will reflect endothelial dysfunction. Vitreal fluorophotometry of different molecular weight fluorescent dextrans will allow quantification of blood- retinal barrier breakdown. Postischemic adherence of fluorescently-labelled leukocytes to venular endothelium will be measured in fresh retinal flat mounts. In vitro , murine retinal endothelial cell cultures from wild type and transgenic animals, including endothelial nitric oxide synthase knockouts, superoxide dismutase knockouts, TNF receptor knockouts, and superoxide dismutase overexpressors, will be used to elucidate mechanisms of ischemic endothelial cell injury. The temporal relationships between intracellular and extracellular production of oxygen free radicals and nitric oxide, size-dependent changes in endothelial barrier permeability, leukocyte-endothelial adherence, and endothelial cell injury will be determined following anoxia- reoxygenation and in response to pharmacologic interventions aimed to inhibit these effects. Elucidation of the oxygen free radical and inflammatory mechanisms underlying the different components of vascular dysfunction and injury in retinal ischemia will be vital to developing therapeutics for ischemic vasculopathies of the retina.